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30-second takeaway
An index return reflects both constituent returns and the weighting rule; a familiar index name does not describe equal exposure to every company.
Learning objective
What you should be able to explain.
Calculate simple capitalization-, equal- and price-weighted returns and explain how weighting changes concentration.
Why this matters
Two indexes holding the same companies can produce different returns and risks.
Vocabulary first
Weight: a constituent’s share of the index. Capitalization weighting: weight based on a specified market-value measure. Rebalance: reset weights under the rules.
- Constituent returns
- Weighting and maintenance rules
- Aggregate return and concentration
Capitalization-weighted indexes commonly use float-adjusted market values, with eligibility and maintenance rules.
For a single period with fixed beginning weights and consistent total-return treatment, multiply each holding’s return by its starting weight and sum the contributions.
Splits, additions, removals, corporate actions and rebalances require rules and divisors.
Follow the mechanism from left to right; each step adds context for the next.
Read the methodology
Capitalization-weighted indexes commonly use float-adjusted market values, with eligibility and maintenance rules. Equal weighting assigns similar weights at specified rebalances. Price weighting gives more influence to higher share prices after adjustments. These are measurement conventions, not independent assessments of business quality or a promise of superior performance.
Weights transmit returns
For a single period with fixed beginning weights and consistent total-return treatment, multiply each holding’s return by its starting weight and sum the contributions. A large constituent can dominate the aggregate even when most companies move the other way. Distinguish average constituent performance from the performance of the weighted index.
Maintenance changes exposure
Splits, additions, removals, corporate actions and rebalances require rules and divisors. Turnover and implementation costs affect a tracking fund differently from a theoretical index. Equal weighting can tilt toward smaller constituents relative to capitalization weighting; it does not remove industry concentration or market risk. Compare methodology, holdings and return type.
Apply the concept.
Hypothetical two-company period: A returns +10%, B −5%. At 80%/20% weights, return is 8% − 1% = 7%. At 50%/50%, it is 5% − 2.5% = 2.5%. With starting share prices $50/$100, a simple price-weighted version has one-third/two-thirds weights and approximately 0% return, excluding corporate actions.
Where reasoning breaks down.
An index rising does not mean every constituent rose, and equal weighting is not the same as equal economic risk.
Can you explain the idea?
Try recalling it in your own words, then compare your reasoning with the lesson explanation.
Try the recall practice →Counterexample: when the shortcut fails
A majority of constituents can fall while a capitalization-weighted index rises if its largest holdings rise enough.
Risk and limits of this model
The fixed-weight one-period example excludes dividends, rebalances, splits and divisor adjustments. Real methodology must be checked.
Put the concept into practice
Compare two fund factsheets for weighting, largest positions, rebalance rules and total-return versus price-return benchmarks.
Go one level deeper
Try a different set of numbers
What is the 80%/20% weighted return if A gains 5% and B loses 10%?
0.8 × 5% + 0.2 × (−10%) = 2%.
MAKE THE IDEA YOURS
Explain it. Try it. Come back to it.
An index return reflects both constituent returns and the weighting rule; a familiar index name does not describe equal exposure to every company.
TRY TO RECALL BEFORE YOU REVEAL
Why can the same constituent list produce different index returns?
Apply it to a second question
What is the 80%/20% weighted return if A gains 5% and B loses 10%?
Compare your reasoning
0.8 × 5% + 0.2 × (−10%) = 2%.
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Use this as a foundation, then verify current rules and product details with primary sources and regulated providers before acting.
Primary sources and further reading
Verify the current details.
Editorial review: 2026-10-14. The examples are hypothetical unless a named historical case is stated. Rules and product details can change; use the relevant source and actual instrument terms.
Connect the concept
Go from explanation to application.
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